Compacted graphite iron, engine cylinder head and vehicle

ABSTRACT

A compacted graphite iron having composition comprising: Fe as a major ingredient, C: of about 3.4˜4.2 wt %, Si: of about 1.5˜2.5 wt %, P: 0.10 wt % or less (not including 0), S: of about 0.10 wt % or less (not including 0), Cr: of about 0.10 wt % or less (not including 0), Mn: of about 0.1˜0.6 wt %, Cu: of about 0.2˜1.6 wt %, Sn: of about 0.1 wt % or less (not including 0), Mg: of about 0.05 wt % or less (not including 0), Mo: of about 0.05˜0.5 wt %, at least one ingredient of V: of about 0.05˜0.5 wt % and Ti: of about 0.05˜0.5 wt %, and other inevitable impurities; an engine cylinder head; and a vehicle.

CROSS-REFERENCE TO RELATED APPLICATION

This application claims under 35 U.S.C. §119(a) the benefit of Korean Patent Application No. 10-2012-0144952 filed on Dec. 12, 2012 the entire contents of which are incorporated herein by reference.

BACKGROUND

(a) Technical Field

The present invention relates to a CGI (compacted graphite iron), which is suitable to be used to vehicle parts and the like such as engine cylinder heads, particularly to a compacted graphite iron having excellent tensile strength and thermo-mechanical fatigue life, an engine cylinder head formed with the compacted graphite iron, and a vehicle equipped with the engine cylinder head.

(b) Background Art

Vehicle parts such as engine cylinder heads are required to have proper tensile strength and thermo-mechanical fatigue life to endure pressure and heat emitted by the engine while driving. Further, there is a need to satisfy demands for improving output and the like.

Particularly, a cylinder head, which is understood by those skilled in the art as a cover covering the top of the cylinder, is installed at a cylinder block with a bolt across a head gasket, and forms a combustion chamber together with a piston and a cylinder. A water cooled type engine head is casted in one piece as a whole cylinder or as several divided cylinders, and includes a water jacket for cooling. An air cooled type engine head is separately manufactured per cylinder, and a cooling plate is installed thereto. Further, there are the combustion chamber and a valve sheet below the cylinder head, or in the upper part thereof, there is a spark plug for a gasoline engine or there are apertures of bores for installation of a glow plug and a spray nozzle and a part for installation of a valve open-close device for a diesel engine.

This cylinder head, which forms part of a combustion chamber, is exposed to higher temperature and higher pressure expansion gas. Accordingly, a material having excellent thermo-mechanical fatigue life should be used to be operated under enough strength and at a certain temperature (200˜500° C.).

On the other hand, aluminum alloy is generally used for a gasoline engine or a light duty diesel engine, but when a heavy duty diesel engine, whose combustion pressure is high, flake-type graphite cast iron or CGI is used as a material for a cylinder head. Among them, the CGI having better tensile strength than the flake-type graphite cast iron is mainly used for the cylinder head of high output heavy duty diesel engine. However, CGI has a demerit of low thermo-mechanical fatigue life due to lower thermal conductivity than the flake-type graphite cast iron, and accordingly, it is very important to secure the thermo-mechanical fatigue life in order to secure competitiveness of the CGI. Therefore, the CGI material for a cylinder head needs improvements of tensile strength and thermo-mechanical fatigue life.

Conventional “CGI cast iron” typically includes iron (Fe) as a major component, carbon (C) of 3.45˜3.55 wt %, silicon (Si) of 2.30˜2.40 wt %, magnesium (Mg) of 0.002˜0.008 wt %, copper (Cu) of 0.10˜0.90 wt %, tin (Sn) of 0.01˜0.09 wt %, chromium (Cr) of 0.03˜0.07 wt %, manganese (Mn) of 0.30˜0.35 wt %, phosphorus (P) of 0.1 wt % or less, and sulfur (S) of 0.1 wt % or less, wherein the carbon equivalent (CE) is 4.3±0.05.”

However, the above composition is not enough to secure the appropriate tensile strength and thermo-mechanical fatigue life. Accordingly, a composition of compacted graphite iron with improved properties is urgently needed.

The description provided above as a related art of the present invention is just for helping understanding the background of the present invention and should not be construed as being included in the related art known by those skilled in the art.

SUMMARY OF THE DISCLOSURE

The present invention has been made in an effort to solve the above-described problems associated with prior art. The present invention provides a CGI (compacted graphite iron), which is suitable to be used to vehicle parts and the like such as engine cylinder heads, particularly to a compacted graphite iron having excellent tensile strength and thermo-mechanical fatigue life, an engine cylinder head formed with the compacted graphite iron, and a vehicle equipped with the engine cylinder head.

In order to accomplish the said object, the compacted graphite iron according to the present invention has composition comprising: Fe as a major ingredient, C: of about 3.4˜4.2 wt %, Si: of about 1.5˜2.5 wt %, P: of about 0.10 wt % or less (not including 0), S: of about 0.10 wt % or less (not including 0), Cr: of about 0.10 wt % or less (not including 0), Mn: of about 0.1˜0.6 wt %, Cu: of about 0.2˜1.6 wt %, Sn: of about 0.1 wt % or less (not including 0), Mg: of about 0.05 wt % or less (not including 0), Mo: of about 0.05˜0.5 wt %, at least one ingredient of V: of about 0.05˜0.5 wt % and Ti: of about 0.05˜0.5 wt %, and other inevitable impurities.

Further, the engine cylinder head of the present invention is manufactured with the composition comprising the compacted graphite iron. Additionally, the vehicle of the present invention is equipped with the engine cylinder head manufactured with the composition comprising the compacted graphite iron.

BRIEF DESCRIPTION OF THE DRAWINGS

The above and other features of the present invention will now be described in detail with reference to certain exemplary embodiments thereof illustrated the accompanying drawings which are given hereinbelow by way of illustration only, and thus are not limitative of the present invention, and wherein:

FIGS. 1 and 2 are graphs showing the results of tensile strength test and fatigue life test of the compacted graphite iron according to one exemplary embodiment of the present invention and a conventional compacted graphite iron.

It should be understood that the appended drawings are not necessarily to scale, presenting a somewhat simplified representation of various preferred features illustrative of the basic principles of the invention. The specific design features of the present invention as disclosed herein, including, for example, specific dimensions, orientations, locations, and shapes will be determined in part by the particular intended application and use environment.

In the figures, reference numbers refer to the same or equivalent parts of the present invention throughout the several figures of the drawing.

DETAILED DESCRIPTION

Hereinafter, the compacted graphite iron, the engine cylinder head and the vehicle according to preferable embodiments of the present invention now will be described in detail with reference to the accompanying drawings.

The terminology used herein is for the purpose of describing particular embodiments only and is not intended to be limiting of the invention. As used herein, the singular forms “a”, “an” and “the” are intended to include the plural forms as well, unless the context clearly indicates otherwise. It will be further understood that the terms “comprises” and/or “comprising,” when used in this specification, specify the presence of stated features, integers, steps, operations, elements, and/or components, but do not preclude the presence or addition of one or more other features, integers, steps, operations, elements, components, and/or groups thereof. As used herein, the term “and/or” includes any and all combinations of one or more of the associated listed items.

Unless specifically stated or obvious from context, as used herein, the term “about” is understood as within a range of normal tolerance in the art, for example within 2 standard deviations of the mean. “About” can be understood as within 10%, 9%, 8%, 7%, 6%, 5%, 4%, 3%, 2%, 1%, 0.5%, 0.1%, 0.05%, or 0.01% of the stated value. Unless otherwise clear from the context, all numerical values provided herein are modified by the term “about.”

The compacted graphite iron (CGI) of the present invention has composition comprises Fe as a major ingredient, C: of about 3.4˜4.2 wt %, Si: of about 1.5˜2.5 wt %, P: of about 0.10 wt % or less (not including 0), S: of about 0.10 wt % or less (not including 0), Cr: of about 0.10 wt % or less (not including 0), Mn: of about 0.1˜0.6 wt %, Cu: of about 0.2˜1.6 wt %, Sn: of about 0.1 wt % or less (not including 0), Mg: of about 0.05 wt % or less (not including 0), Mo: of about 0.05˜0.5 wt %, at least one ingredient of V: of about 0.05˜0.5 wt % and Ti: of about 0.05˜0.5 wt %, and other inevitable impurities.

The CGI of the present invention has very excellent tensile strength and thermo-mechanical fatigue life compared with previous materials having a CGI level. Accordingly, the CGI material of the present invention can be used for high output cylinder head.

In the CGI of the present invention, thermo-mechanical fatigue life is improved by adding Mo, V and Ti of chemical ingredients, and excellent tensile strength is secured by strengthening perlite structure as a matrix by adding Sn and by securing fine perlite structure by adding Mo and regulating the upper limit of the Si content.

The functions and contents of the ingredients contained in the CGI according to the present invention are as follows.

The carbon (C) is an essential atom for forming the high-temperature stable phase, and is preferably contained at the amount of about 3.4˜4.2 wt % in view of fluidity reduction and primary graphite crystallization.

The silicon (Si) is contained at the amount of about 1.5˜2.5 wt %. The silicon contributes to graphite crystallization, and therefore, it is needed to be contained at the amount of about 1.5 wt % or more. The silicon is a representative atom interrupting perlite refining. Therefore, in order to improve tensile strength through the perlite refining, the silicon content should be as low as possible, and when the silicon content is high, more perlite refining atoms (Mo and the like) should be added. Accordingly, in the present invention, the silicon content was determined to be about 2.5 wt % or less, which is the level which would not interrupt the perlite refining.

Particularly, the silicon content was regulated to be about 2.5 wt % or less, the level not excessively demanding the addition of perlite refining atoms (Mo and the like). In the case of molybdenum, castability and machinability are decreased when its content is over 0.5 wt %. Accordingly, the molybdenum was added to improve tensile strength by refining the perlite structure, and the content silicon (Si), which attributes to graphite crystallization, was optimized to the level not interrupting the perlite refining at the same time. On the other hand, the phosphorus (P) is contained in the amount of 0.10 wt % or less. The phosphorus forms steadite and affects to brittleness. Accordingly, it is preferred to limit the amount to about 0.10 wt % or less.

The Sulfur (S) is contained at the amount of about 0.10 wt % or less. The sulfur forms MnS by being combined with Mn so as to improve machinability, but affects to brittleness when the amount is over 0.10 wt %. Accordingly, it is preferred to limit the amount to 0.10 wt % or less.

Further, the chromium (Cr) is contained at the amount of about 0.10 wt % or less. When the amount is over 0.10 wt %, machinability may be decreased. Accordingly, it is preferred to limit the amount of chromium (Cr) to 0.10 wt % or less.

The manganese (Mn) is contained at the amount of 0.1˜0.6 wt %, and improves tensile strength by strengthening perlite matrix. However, when the amount is over 0.6 wt %, machinability may be decreased. Accordingly, it is preferred to use the manganese (Mn) in the amount of 0.1˜0.6 wt %.

On the other hand, the copper (Cu) is contained at the amount of about 0.2˜1.6 wt %. The copper, an atom making perlite matrix, strengthens the matrix and improves tensile strength. However, when the amount is over 1.6 wt %, the effects are saturated. Accordingly, it is preferred to use the copper (Cu) at the amount of about 0.2˜1.6 wt %.

The tin (Sn) is contained at the amount of about 0.1 wt % or less. Because it has higher effects to on the perlite matrix than the copper, tensile strength can be improved. However, when the content is over 0.1 wt %, the effects may be saturated. Accordingly, the tin (Sn) content is limited to about 0.1 wt % or less.

The magnesium (Mg) is contained at the amount of about 0.05 wt % or less. The magnesium is an atom required to make graphite form into compacted graphite, which is an intermediate form of flake form and spheroidal form. However, when the magnesium content is over 0.05 wt %, the graphite is spheroidized. Accordingly, it is preferred to limit the magnesium (Mg) content to about 0.05 wt % or less.

Particularly, the molybdenum (Mo) is contained in the amount of 0.05˜0.5 wt %. The molybdenum is an atom refining the perlite matrix structure, and it should be contained because it improves tensile strength by refining the perlite structure formed by the copper (Cu) and the tin (Sn). Further, it forms Mo Carbide by combining with the carbon (C), and then sometimes forms the high temperature stable phase improving thermo-mechanical fatigue life. However, when its content is over 0.5 wt %, castability and machinability are decreased. Accordingly, it is preferred to use the molybdenum (Mo) in the amount of about 0.0˜0.5 wt %.

As described above, the silicon (Si) is contained in the amount of about 1.5˜2.5 wt % at the same time, thereby it attributes to graphite crystallization and is regulated to the level not interrupting the perlite refining.

On the other hand, at least one atom of vanadium (V) and titanium (Ti) should be contained in the amount of 0.05˜0.5 wt %. This atom forms V/Ti Carbide by combining with carbon (C), and forms high-temperature stable phase improving thermo-mechanical fatigue life. But, because castability and machinability are decreased when the amount of at least one atom of vanadium (V) and titanium (Ti) is over 0.5 wt %, it is preferred to limit the amount of at least one atom of vanadium (V) and titanium (Ti) to 0.05˜0.5 wt %.

Effects of the compacted graphite iron of the present invention can be confirmed by comparing with Comparative Example, and specific compositions of Comparative Example and Examples are as follows.

TABLE 1 Composition (wt %) C Si P S Cr Mn Cu Sn Mg Mo V Ti Fe Comp. 3.78 2.02 0.03 0.02 0.03 0.32 0.82 0.04 0.02 — — — Rem. Example Example 3.67 1.95 0.04 0.03 0.04 0.29 0.79 0.09 0.02 0.3 — — Rem. 1 Example 3.71 2.03 0.02 0.02 0.02 0.33 0.83 0.09 0.02 0.2 — 0.3 Rem. 2 Example 3.68 2.01 0.02 0.03 0.03 0.31 0.80 0.09 0.02 0.2 0.3 — Rem. 3

The composition of Examples 1, 2 and 3 are the composition of the compacted graphite iron suggested by the present invention, and the composition of Comparative Example is the composition of the compacted graphite iron having 400 MPa level of tensile strength suggested as the conventional material for engine cylinder heads.

Tensile tests were conducted according to KS B 0802 (tensile test method of metallic materials). No. 8 test specimen was used as a test specimen according to KS B 0801 (tensile test specimen of metallic materials). As a result, as shown in FIG. 1, Examples 1, 2 and 3 showed higher tensile strength, which was improved 10% or more than Comparative Example, and it is considered perlite matrix strengthening effect by Sn and perlite matrix refining effect by Mo. Namely, these were resulted from perlite strengthened by adding more Sn than before, and from perlite refined by newly adding Mo.

On the other hand, thermo-mechanical fatigue life was conducted by controlling temperature condition to room temperature ˜450° C., stress strain rate to ±0.75% at 200 sec/cycle. As shown in FIG. 2, it was confirmed that thermo-mechanical fatigue life of each of Examples 1, 2 and 3 was increased, 20% or more, than Comparative Example, and it is considered effect by generation of Mo/V/Ti Carbide, which is high-temperature stable phase, according to adding Mo, V and Ti.

According to the compacted graphite iron, the engine cylinder head and the vehicle having the constitution described above, Mo/V/Ti Carbide as high-temperature stable phase can be formed by adding Mo, V and Ti atoms improving thermo-mechanical fatigue life, by adding Sn atom improving tensile strength by matrix strengthening, by adding Mo atom improving tensile strength by perlite refining, and by optimizing other alloy ingredients, compared with the conventional CGIs. Particularly, by selecting Si in charge of graphite crystallization and Mo in charge of perlite refining within a balanced range, both of the two effects can be obtained.

Through this, a CGI having improved tensile strength of 10% or more and thermo-mechanical fatigue life of 20% or more against the conventional CGI materials can be obtained. Further, the CGI is excellent on tensile strength and thermo-mechanical fatigue life by forming good compacted graphite and fine perlite structure against the conventional CGI, and therefore, it can be suitably used for engine cylinder heads that requires power improvement.

The invention has been described in detail with reference to preferred embodiments thereof. However, it will be appreciated by those skilled in the art that changes or modifications may be made in these embodiments without departing from the principles and spirit of the invention, the scope of which is defined in the appended claims and their equivalents. 

What is claimed is:
 1. A compacted graphite iron having composition comprising: Fe as a major ingredient, C: of about 3.4˜4.2 wt %, Si: of about 1.5˜2.5 wt %, P: of about 0.10 wt % or less (not including 0), S: of about 0.10 wt % or less (not including 0), Cr: of about 0.10 wt % or less (not including 0), Mn: of about 0.1˜0.6 wt %, Cu: of about 0.2˜1.6 wt %, Sn: of about 0.1 wt % or less (not including 0), Mg: of about 0.05 wt % or less (not including 0), Mo: of about 0.05˜0.5 wt %, at least one ingredient of V: of about 0.05˜0.5 wt % and Ti: of about 0.05˜0.5 wt %, and other inevitable impurities.
 2. An engine cylinder head manufactured with the composition comprising the compacted graphite iron according to claim
 1. 3. A vehicle equipped with an engine cylinder head manufactured with the composition comprising the compacted graphite iron according to claim
 1. 